It is often necessary or desired to provide a precision coating of a particular substrate such as a glass panel. For example, in the video electronics industry it is often desired to coat panels which will serve as flat panel displays (FPD) to be incorporated into television sets, computer monitors and the like. It is important in such applications to ensure the accuracy and consistency of coating thicknesses across the panel.
A commonly employed method of coating flat panel displays is to have a stationary head extruding fluid at a particular rate over linearly moving panels. Using such a configuration, the coating consistency is dependent upon a number of parameters such as the gap between the head and the panel surface, the variation in this gap as the panel moves, the dimensional consistency of the panel, the mechanical tolerances of the extrusion orifice or slot, the pump characteristics, and the presence of gas or air bubbles in the coating material. Additional factors affecting variation in the thickness in the coating across the area of the panel will be the consistency of fluid flow rate through the extrusion head, and the consistency of linear velocity of the panel under the head as well as the ability to maintain steady movement, as measured in each of the x, y and z planes, of an often large substrate. The above all represent problems in the art.
In the context of this discussion, the length of the dispensing head refers to the span of the head, generally in a direction perpendicular to the coating direction. This “length” of the dispensing head may correspond to the same direction as the width of the substrate to be coated, since the dimension of the substrate concerned may in fact be the shorter of the two horizontal dimensions of said substrate. In addition to the considerations of distance between various key elements is the issue of vertical flexing in a extrusion head across its own length. The extent of this problem will depend upon the nature of the support structure for the head as well as the length and density of the head structure. To the extent that such vertical flexing is present, it presents the problem of variation in height between the head and the panel along the length of the head.
The moving panel approach requires a large footprint for the overall mechanism because there must be at least enough space set aside for the full area of the panel on both sides of the fluid extrusion means. There is also a need for leveling the panel throughout its travel underneath the extrusion means. Further, the disadvantages of a large footprint requirement and leveling issues increase as the size of the panel increases. Therefore, it is problem in the art that the system footprint must be at least double the area of the panel to be coated. It is also a problem in the art that there could be variation in height between the head and the panel along the length of the panel.
In order to avoid dripping or smearing coating material which has gathered around the extrusion head after a coating operation, it is often necessary to clean the extrusion head before a new coating operation begins. In the prior art, cleaning of extrusion mechanisms is usually accomplished manually, potentially leading to inconsistent results and disruption and delay of the coating operations. Therefore, it is a problem in the art that manual cleaning operations are inconsistent and unreliable.
In order to ensure that coating material is applied consistently and evenly right from the start of the coating operation, it is desirable to ensure that a bead is fully and properly formed at the extrusion head prior to starting the coating process. A problem in the prior art exists with respect to properly priming fluid extrusion heads so as to ensure that a proper bead is formed prior to extruding fluid over the panel, and that a consistent rate of coating fluid flow is thereafter achieved as the full area below the extrusion head must be maintained open for the passing of a substrate thereunder, thus making it difficult to provide any priming mechanism.
Generally, in prior art coating systems, there is a single pump mechanism located remotely from the extrusion head with appropriate fluid conducting means leading from the pump to the head. The use of a single pump, while perhaps economical, makes it difficult to precisely control fluid flow at the extrusion head. Specifically, it may be difficult to start and stop at precisely defined moments and to establish the precise fluid flow rate desired. Some prior art systems have used two pumps.
In the prior art, the fluid delivery means, including fluid supply, pumps, and fluid extrusion head assembly were all part of a single integrated coating apparatus assembly. As such, when it was necessary to change coating fluids, or perform other operations on the fluid delivery means, the entire coating apparatus would be idled. Fluid changeover operations include time consuming tasks such as cleaning all tubing, pumping mechanisms, and essentially all surfaces where residue of the previous coating material could be present. This thoroughness is necessary because of potentially dangerous chemical reactions between two different coating materials to be used in succession, and the possibility of cross-contamination between materials used in different processes. The idle time for the coating apparatus is expensive and wasteful given that mechanisms unrelated to the fluid delivery system are idled by the operations necessary for fluid changeover. Accordingly, a need exists in the art for a system and method wherein a chuck assembly adapted to position and hold substrates to be coated as well as other components and materials used in the coating process, but not part of the fluid delivery system are not left idle during fluid delivery system cleaning operations.
In prior art systems, variation in the height of the extrusion head with respect to the panel can cause breaking of the coating bead and variation in coating thickness. The causes of such height variation include part dimension variation, part placement error, and gradual drift in machine dimensions over time. Accordingly, there is a need in the art for a system and method for ensuring constant extrusion head height over the panel being coated.
Accordingly, a need exists in the art for a system and method for providing a uniform coating of a desired thickness on a relatively large substrate, including panels of various shapes and sizes, while providing efficient use of a coating material.
A still further need exists in the art for a system and method for coating substrates which will minimize the footprint of the coating system.
A still further need exists in the art for a system which is adaptable to very large substrate sizes.
A still further need exists in the art for a system in which a constant extrusion head gap is maintained irrespectively of flex associated with the use of a linear extrusion head.
A still further need in the art exists for a cleaning station whose functions are easily accessible to a fluid dispenser such as an extrusion head at appropriate times such as between coating operations.
A still further need in the art exists for a priming station which can be accessed easily by a fluid dispenser such as an extrusion head at appropriate times such as between coating operations.
A still further need in the art exists for more precisely controllable flow of coating material at a fluid dispenser such as an extrusion head.